This invention relates to a chain tensioner for maintaining proper tension in a power transmitting chain. The tensioner has utility, for example, in maintaining tension in the timing chain of an internal combustion engine, where the chain transmits power from a crankshaft sprocket to one or more camshaft sprockets.
In a conventional tensioner, a plunger, protruding from a housing, exerts a force on the back of a pivoted tensioner lever having a shoe in sliding contact with the slack (return) run of a chain in order to maintain tension in the chain. The plunger of the tensioner engages the lever at a location remote from the pivot axis of the lever.
A conventional ratchet type tensioner 500, as shown in FIGS. 9 and 10, comprises a plunger 510, slidable in a hole 531 formed in a housing 530. The plunger protrudes from the housing, and is biased in the protruding direction by a spring 520. A pawl 550, pivotally supported by a pin 540 on the housing 530, is biased by a spring 560 so that its pawl teeth 551 engage teeth 511 formed on the outer periphery of the plunger.
When the plunger 510 protrudes from the housing 530 as a result of loosening of the chain, and is thereafter subjected to an impact force from the tensioner lever, the ratchet mechanism blocks retraction of the plunger by engagement of the pawl teeth 551 with the ratchet teeth 511 on the plunger.
When the ratchet operates to prevent retraction of the plunger, the ratchet pawl 550 applies a sideways force to the plunger, which results in eccentric loads, indicated by arrows F1 and F2 in FIG. 10, exerted by the plunger on the hole 531. Especially when the housing 530 is formed of aluminum, these loads can result in significant localized wear.
In the conventional ratchet-type tensioner 500, when the plunger 510 is subjected to excessive impact force from the tensioner lever, the impact force is concentrated on the ratchet teeth 511 and the pawl teeth. The impact force produces significant wear of the tips of these teeth, and failures due to excess wear or breakage of the teeth can occur.
Furthermore, in the conventional ratchet-type tensioner 500, the control of tension in a traveling chain depends on engagement between the ratchet teeth 511 and the pawl teeth 551. To attain smooth engagement of these teeth, high accuracy in the diameter and mounting position of the pivoting pin 540 are required. Troublesome problems have been encountered in the production of the pin, and also in the production of a pawl with an accurately positioned pin-receiving hole having the proper diameter.
Among the objects of the invention are the solution of the above-described problems, and the provision of a chain tensioner which can disperse and reduce the load applied to the plunger, which exhibits superior wear resistance, and which can reduce the requirement for high accuracy in production of components and assembly thereof.
The chain tensioner in accordance with the invention is similar to a conventional chain tensioner in that it comprises a housing having a plunger-receiving hole with an opening, a plunger slidably fitting into the plunger-receiving hole and protruding from the opening thereof in a protruding direction, and a spring biasing the plunger in the protruding direction.
The chain tensioner differs from the conventional chain tensioner in that it comprises toothed racks formed on opposite sides of the plunger, and a pair of wedge-shaped cams on opposite sides of the plunger, each cam being disposed adjacent one of the respective toothed racks, each the cam having teeth Mockingly engageable with teeth of the adjacent rack to prevent retracting movement of the plunger relative to the cams, and each cam having an oblique surface for engagement with a seat. The oblique surface of each cam faces outwardly away from the plunger, and the shape of each oblique surface is such that the oblique surface has an outer end disposed farther than an inner end thereof along the protruding direction of the plunger. Oblique cam seats are formed in the housing on opposite sides of the plunger, and the oblique surfaces of the cams conform to, and are in engagement with the oblique cam seats and slidable thereon to an extent such that the teeth of the cams can be brought into and out of locking engagement with the teeth of the racks. A spacer is disposed on the housing adjacent the opening, and a second spring, engaged with the spacer and the cams, urges the cams in a direction opposite to the protruding direction of the plunger so that the cams are urged into locking engagement with the toothed racks.
In a preferred embodiment, the plunger and housing define an oil chamber within the housing, and a hydraulic valve mechanism is provided on the housing for allowing one-way flow of oil from an external supply of oil under pressure into the oil chamber to apply an additional force biasing the plunger in the protruding direction.
In a preferred embodiment of the invention the plunger is rotatable in the housing, so that engagement between the rack and the wedge-shaped cams can be forcibly released.
If the plunger-biasing spring is interposed between the flange and the spacer, it is more easily installed, and can be used to hold the spacer against the housing.
In an alternative embodiment, the plunger-biasing spring is disposed in the plunger-receiving hole and is interposed between an end of the plunger located within the hole and an end of the hole. In this case, the spacer is preferably screwed into housing.
The chain tensioner of the invention may be either the inner mounting type or an outer mounting type.
The teeth on the plunger racks and the cams can take various forms so long as they are capable of exhibiting a ratcheting action allowing the plunger to move in the protruding direction but preventing retraction thereof. The protrusion biasing spring causes the plunger to move rapidly in the protruding direction when the chain becomes loose, thereby reestablishing proper tension.
The cam biasing spring, which is loosely fitted onto the plunger, biases the wedge-shaped cams against their seats, which, in turn, exert a radially inward force on the cams so that they are urged into engagement with the plunger. However, when the plunger moves in the protruding direction, it draws the cams with it in the protruding direction, and at the same time its teeth, in cooperation with the teeth of the cams, urge the cams radially outwardly, holding the cams against their seats until the teeth disengage. At the instant when the teeth become disengaged the plunger will move forward by one tooth of the rack, and the cams are immediately reengaged with the rack, to prevent retracting movement of the plunger, and maintaining proper chain tension.
The use of cams on opposite sides of the plunger distributes the force applied by the chain to the plunger, thereby avoiding concentrated loads on the housing. Moreover, the cams distribute the force symmetrically, avoiding eccentric loads as in the case of the conventional tensioner.
Furthermore, in the chain tensioner according to the invention, even when the plunger is subjected to excessive impact force from the chain through a tensioner lever, the plunger is prevented from retracting so that proper chain tension is maintained.